1. Field of the Invention
This invention relates to a two stage hydrocracking process for producing multiple grades of lube oil base feed stock.
2. Description of Related Art
Hydrocracking is typically used for conversion of relatively heavy compounds in a feedstock into lighter compounds. These processes utilize different reactor configurations including single and dual stage systems that operate in once-through or recycle schemes. Uncertain operations additional useful product recovered includes a lube oil base feedstock, whereby once-through flow schemes target a fixed conversion and viscosity index. Thus, the product lube oil base feedstock necessarily is of a single grade with the fixed viscosity index.
As demand for different grades of lube oil base feedstock increases, the ability of the hydrocracking unit to make multiple grades of lube oil base feedstock from unconverted oil produced becomes challenging. A two stage hydrocracking configuration has the ability to achieve different qualities of unconverted oil; based on the fact that both the conversion on the first stage and the conversion per pass on the second stage can be adjusted for a fixed overall conversion, and to achieve the targeted unconverted oil quality. Since the other fuel products co-produced need to meet clean fuel standards; the limitation for single stage operation can be overcome by two stages. However the fractionation system needs to be independent in order to achieve the aforesaid target. Thus traditional hydrocracking flow-schemes are limited on account of their configuration and if the flexibility is required, then atypical fractionation schemes are required. This increases complexity of the plant design and also increases both capital and operating expenditures to achieve the desired targets.
U.S. Pat. No. 6,379,535 (Hoehn) discloses a divided wall column that is used as a HPNA rejection chamber. Hoehn is not concerned with flexibility of a divided wall column as provided in the present disclosure for production of multiple grades of lube oil base feedstock within a hydrocracking unit.
US Patent Application 2011/0132083 (Umansky) discloses the combination of a single stage hydrocracking with dewaxing, where a divided wall column acts as a separator between the hydrocracked products and dewaxed products.
A need remains for an effective and economical method to produce multiple grades of lube oil base feedstock within a hydrocracking unit.